It long has been known that certain metals and metallic alloys when heated to particular temperatures may be formed into cavities by relatively low gaseous pressures or diffusion bonded. For example, titanium alloys of 40% aluminum and 60% titanium when heated to a temperature of about 956.degree. C. may be so processed. To avoid various problems associated with such forming, it is common to establish and maintain an inert gaseous atmosphere about the material being processed when it is at elevated temperatures, using an inert gas such as nitrogen, argon or the like.
In order to maintain the desired atmosphere about the hot metal components during the superplastic forming diffusion bonding operation, the entire forming apparatus necessarily must be enclosed in an envelope or container that is sealed to avoid contamination by outside gasses, such as air, oxygen, hydrogen, carbon dioxide and the like. However, due to the elevated tempertures required in the forming process, it is difficult to maintain seals. Organic seals have been tried but none has been found which can withstand the temperatures involved in the forming operation. Metallic seals, on the other hand, heretofore have not been able to provide an adequate seal because of warping of the adjacent surfaces against which the seals must operate. In some practical instances, the very size of the material to be formed and the ultimate product produced severely amplify this problem.
Heretofore, it has been the practice to create an envelope about the die mechanisms used in the forming process by welding or otherwise continuously connecting components or juxtapositioned peripheral edges of the envelope. This is an expensive and time consuming procedure and necessitates a cutting operation to remove the envelope from the balance of the hardware to gain access to the dies and the formed part after the process is complete.
Rotating machinery such as steam turbines commonly used counterflow labyrinth seals in a hot high pressure environment. However, these seals generally are of the loose contact type because of the necessity that the parts move with respect to each other during operation.